1. Field of the Invention
The present invention generally relates to a detection system for an alcohol sensor failure in an internal combustion engine. More specifically, the present invention relates to such an alcohol sensor failure detection system that can detect the failure of the alcohol sensor with high accuracy.
2. Description of the Background Art
An alcohol internal combustion engine has been proposed, such as disclosed in Japanese Patent First Publication No. 56-98540, wherein a gasoline-alcohol mixed fuel is used in place of a pure gasoline fuel. Generally, methanol or ethanol is employed as an alcohol component.
In this publication, an alcohol sensor is provided for monitoring alcohol concentration contained in the mixed fuel to control a fuel injection amount and an ignition timing based on a monitored alcohol concentration. Naturally, the fuel injection amount and the ignition timing are variable depending on the alcohol concentration because an octane value contained in the gasoline-alcohol mixed fuel and the pure gasoline fuel is different from each other. An oxygen sensor (O.sub.2 sensor) is further provided for monitoring oxygen concentration contained in an exhause gas so as to perform a FEEDBACK or CLOSED LOOP control of the fuel injection amount to maintain an air/fuel ratio of an air/fuel mixture at a stoichiometric value in a predetermined engine driving range.
In the alcohol internal combustion engine as noted-above, since the fuel injection amount and the ignition timing are controlled based on the monitored alcohol concentration, when the alcohol sensor malfunctions, the fuel injection amount and the ignition timing can not be properly controlled resulting in the unstable driving condition of the engine.
For overcoming the above-noted defect, there has also been proposed a fail-safe control which is executed when the failure of the alcohol sensor is detected, so as to maintain the stable driving condition of the engine.
Accordingly, the detection of the malfunction of the alcohol sensor is very important for properly operating the engine. Generally, the alcohol sensor failure has been detected by detecting an abnormal output voltage of the alcohol sensor which is generated due to short or open circuit of a harness in the alcohol sensor. However, a reliable detection of the alcohol sensor failure can not be ensured using the output voltage of the alcohol sensor since the failure of the alcohol sensor does not always cause the alcohol sensor to output the abnormal voltage which deviates from a predetermined reference voltage range, particularly when a degree of the failure is relatively small.